Today: Applied Materials is fourth company in top 50 of Silicon Valley tech firms and fifth SV150 company to make a change in its top role this summer. Also: Valley tech stocks bounce back slightly from Thursday's weakness, as Apple (AAPL) closes above $500 for first time since January.

The third quarter has been rough on Silicon Valley CEOs: Just halfway through the three-month period, during which companies typically detail their performance in the first half of the calendar year and reveal forecasts for the second half, five of Silicon Valley's largest technology companies have made a change at the CEO position.

Applied Materials was the latest to join the trend, announcing Thursday afternoon that Michael Splinter would be replaced almost exactly 10 years after taking the helm at the Santa Clara chip-equipment provider. While Splinter will stick around as executive chairman of the company, Gary Dickerson will take over after serving as president since the semiconductor company he previously helmed, Varian, was acquired by Applied Materials in 2011.

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Analysts said that the move reflects positively on Applied Materials, which is caught up in the same semiconductor weakness that has been felt across the valley that gained its name from chips' main ingredient. D.A. Davidson analyst Thomas Diffely said the move "should reinvigorate investor interest," and IC Insights analyst Bill McClean deemed the move unsurprising, with the weak semiconductor market prompting the board to "get some fresh ideas or new blood in there." Shares rose Friday, gaining 1.9 percent to $15.62 after Applied Materials also announced a return to profitability.

While the chip business has been rough, it is not alone in leadership changes in Silicon Valley, with nearly all sectors affected by the summer's wave of changes. If recent history is any indication, Applied Materials' stock bump may not last, as the other four companies have experienced a decline since their moves were announced.

-- The quarter kicked off with a big change at social-gaming company Zynga, which removed founder Mark Pincus from the CEO role and replaced him with the former head of Microsoft's Xbox division, Don Mattrick. Mattrick immediately began remaking the company, announcing in its quarterly earnings report that the San Francisco company would not seek a license to offer real-money gaming in the United States, which many analysts saw as Zynga's last opportunity for growth large enough to justify its $10 initial public offering price. This week, Mattrick continued to clean house, announcing the departure of three top executives as he seeks to take "layers out of the executive rank to get senior leaders closer to important product initiatives," according to a blog post. While the stock got an immediate bounce from the arrival of the respected tech exec, Zynga closed Friday lower than the $3.07 level it closed at on the day he was brought on, at $2.90.

-- Polycom had the most controversy in its CEO dismissal, announcing that Andrew Miller was leaving the videoconferencing company after the board had discovered irregularities in his expense reports. The loss of the popular Miller, replaced on an interim basis by Kevin Parker, was "a huge loss," Enderle Group principal analyst Rob Enderle said. But analyst Michael Tchong noted, "Their brand is so strong that Polycom is a fixture in just about every conference room." Polycom stock dipped immediately following the news and has yet to recover, however: Shares closed before the announcement at $11.18 and finished Friday at $9.98.

-- On the same day as Miller's dismissal, Sunnyvale-based Juniper announced that its CEO, 52-year-old Kevin Johnson, would retire as soon as a suitable replacement was found. A retirement for an executive so young seemed odd, but later news may have pointed in a different direction: The networking company disclosed in a regulatory filing that it is being investigated for overseas practices. Juniper stock has slipped 4.2 percent in the wake of the retirement and subsequent disclosure, closing Friday at $20.44.

-- While the other four companies to dismiss their CEOs are among the top 50 public tech companies in terms of revenue, the smaller companies are not immune from turnover. Hayward's Solta Medical, No. 134 on the SV150, announced the resignation of its CEO Steven Fanning, while it also revealed disappointing quarterly earnings. The stock has steadily declined since the dual announcements, ending Friday at $2.04, down 18.7 percent since Fanning left.

There is likely to be at least one more new CEO in Silicon Valley this quarter, though his or her predecessor has already been shown the door: Redwood City gaming giant Electronic Arts (ERTS) has been operating under an interim CEO since its leader left in March.

SV150 market report: Tech stocks bounce back, Apple closes above $500

After Wall Street suffered Thursday, including Silicon Valley tech stocks, Friday's trading session provided a bit of a bounceback, with the SV150 experiencing a slight uptick despite small losses for the major stock indexes that closed out the worst week for Wall Street this year.

Silicon Valley's clean-energy sector had a good day: Tesla Motors (TSLA) gained 1.7 percent to $142, SolarCity moved 4.1 percent higher to $37.06, and SunPower (SPWRA) increased 3.3 percent to $21.35. Facebook had its first positive trading day of the week but still ended Friday lower than its IPO price, closing with a 1.4 percent gain to $37.08.

And the widely watched Standard & Poor's 500 index: Down 5.49, or 0.33 percent, to 1,655.83

Check in weekday afternoons for the 60-Second Business Break, a summary of news from Mercury News staff writers, The Associated Press, Bloomberg News and other wire services. Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/mercbizbreak.